The term "cling film" as used in the specification and claims may be defined as a polymeric film that adheres directly to a substrate having a smooth surface such as glass, plastic, paper, wood, metal, etc., without the use of glues or adhesives. Cling films can be readily removed from the substrate and reapplied to the same or a different substrate a number of times. Cling films are usually highly flexible polymeric films that cling to a variety of surfaces by cohesion or as a result of a vacuum that is formed when the flexible smooth film is applied to a smooth, rigid surface such as glass, and in some cases to a light matte or textured surface such as in the front of a refrigerator.
Stretch cling films having a wide variety of applications have been known and used for many years for bundling packaged food and other goods. One application of particular interest for the stretch cling films is the bundling of goods for shipping and storage, for example, the bundling of large rolls of carpet, fabric, or the like. Another important and widely used bundling application is the containment and unitization of pallet loads. In these applications, a wrapping cling film is stretched tightly around an article or group of articles, and self-sealed in the stretched condition. In these application, it is essential that the films have cling properties in the stretched state. Good printability is not a requirement.
The present invention is not concerned with stretch cling films, but rather, films which have cling properties in the unstretched state. The cling films of the present invention, particularly when ink-imprinted with a message or design are useful in advertising such as in point of purchase advertising. The unique ability of the cling film to stick to a surface without adhesive is desirable for constant changing of point of purchase advertising. For example, cling films containing a message or design are particularly useful when there is a need for a decal on a product for promotional reasons. Examples include the fronts of refrigerators or microwave ovens where advertising helps to explain the various features of the product. Cling films also are used for advertising products on storefront windows, freezer cases and walk-in coolers. Cling films which have high clarity or can be totally opaque are particularly useful for such purposes.
An important advantage of using a cling film as opposed to an adhesive-backed decal in any application is that the cling film can be easily removed from a substrate without leaving residue.
A recent high volume use of cling film (in particular, vinyl film) has been in the "oil lube sticker" market. When an oil change is obtained at a commercial establishment, the clear cling film sticker is often applied in the upper left-hand corner of the automobile window that informs the owner of the vehicle when the oil in the vehicle should be replaced. The sticker can be easily removed and replaced with a new sticker.
Most of the cling films which have been used are vinyl chloride films (PVC) which have been used extensively because such films are highly flexible, particularly when they contain additives such as plasticizers and/or tackifiers. Generally, the vinyl cling films are calendered films as opposed to extruded or cast films. The films generally comprise 45% to 50% by weight of PVC resin, stabilizers and pigment, and the remainder is a plasticizer such as dioctyl phthalate (DOP). The thicknesses of the film range from about 0.004 to 0.014 inches with both surfaces being double polished. The cling film is generally supplied with a release liner which is usually paper or paperboard stock either with a castcoated or polycoated surface on one side. The cast coating is a clay, and the backside is either free of coating or has a light starch sizing.
It has been known for many years that tackifiers and plasticizers can be added to a polymer to give the polymer the necessary tackiness or cling to adhere to other smooth surfaces. Examples of tackifying additives include surfactants, polybutene, terpene resins, hydrogenated rosins, alkali metal and glycerol stearates and oleates and rosin esters. The use of tackifiers has disadvantages. While tackifiers are known in the art to enhance cling in an olefin cling film, the presence of the tackifier may cause blending difficulties during film manufacture and/or adversely affect optical properties of the film. Another disadvantage of tackifiers, plasticizers and surfactants is that they have a tendency to migrate to the surfaces of the film, and although it has been suggested that such migration to the surfaces enhances the cling properties of the film, the presence of the tackifiers and plasticizers on the surfaces of the films generally reduces the ability of the film to be ink-imprinted. The presence of the tackifiers at the surface also can cause damage to the substrate to which the cling film has been attached.
In addition, the presence of plasticizers and tackifiers in cling films such as vinyl cling films has, in recent years, been the subject of protest by various environmental groups who are opposed to the use of halogenated polymers such as polyvinyl chloride and to the use of certain additives such as the dioctyl phthalate plasticizer commonly used in PVC films.
Accordingly, there is a need for cling films which can replace the vinyl films which can be reapplied after being peeled from a smooth surface, and which do not require the use of plasticizers or tackifiers.